The first objective of these investigations is to describe the biochemical properties of and the relationships among the five distinct components of the prothrombin activation process. As a consequence, the ultimate objective which is to provide a rational chemical mechanism for this complex multicomponent process and its regulation may be realized. The specific role of phospholipids in organizing the protein components through adsorption of the protein onto the surface of the lipid aggregates will be defined. The specific peptide bounds which are cleaved during conversion of factor X to its catalytically active form and which also results in the transfomation of the factor X molecule from a form which does not bind to lipid to a form which does will be defined. Similarly, those changes in prothrombin will be determined which both yield thrombin and result in desorption of the prothrombin from the lipid surface. Adsorption isotherms relating the individual protein concentrations, the lipid surface characteristics and solution composition will be determined as these specifically characterize the lipid-protein binding. A detailed comparison of the rates of prothrombin activation in solution with activated factor X alone, with activated factor X phospholipid dispersions will be made.